Significant villains in Jason's story
If you think of the rogues that the Red Hood fought, the first thing that comes to mind is...
The Joker - no need to explain here.
The Black Mask - Jay brought him to the point where Roman agreed to kill his men so that the Hood would leave him alone. Or maybe you can remember the one where Roman wanted to make Jay his successor.
Cheer - the story itself may be controversial, but the thief himself turned out to be a good one, and the thematic one is very suitable for Jay as an opponent.
Mr. Freeze - here, in fact, they already have three confrontations, one during Under the Red Hood, the second during the Night of the Owl Court, and the third during the Cheeer story.
Ma Gunn - she tried to make Jason her minion, it failed, and then we find out that she's his Grandmother. It could have worked, but they decided to make her good. A grandmother who shot salt at her students.
Solitari - Lobdell almost managed to write his Killgrave, but as always, the key here is “almost”, but this villain has potential, just give him a good writer.
Essence - they seem to have been in love with each other, but they both tried to kill each other, stupidly not understanding the situation.
Underbelly - I mention it for two reasons. He cannot be killed, and he was mentioned by someone other than Lobdell
Susie Su - a comedic villain
Fellipe Garzones - One issue, but he made a huge impact on Jason's character and his story.
Penguin and Two-Face are also worth mentioning because one of them is responsible for Willis Todd's “death”. But then again, it's not clear which one of them did it, given that we have all the canon now... so it's just a mention.
Who else can be added to this list? Just if we add other worlds, I can also mention Shriek from the White Knight, but again, another world, so just a mention.